There is a cost to dynamically loading each css file and then applying the styles over the DOM for each file. I would favor just doing what email does: just one mail.css with @import calls in it. Then have the r.js optimizer inline them into one file. I expect that will be just as good of a perf result, as it is just doing one disk operation and one style hit. The make_gaia_shared.js script will make sure support assets are kept around (it checks for style/*.css for files to process).